A typical example of depositing metal on a substrate with the use of a focused ion beam is an ion beam-assisted CVD. This type of CVD method consists of introducing gaseous organic metal into a vacuum prepared immediately before a target, and irradiating the gas with a focused ion beam so as to decompose the gas into metallic atoms which are deposited on the target.
However, the ion beam-assisted CVD is disadvantageous in that the quality of the resulting metallic film is spoiled with unwanted atoms contained as impurities.
The inventors have found the effectiveness of another method which consists of generating ions from a source, separating a desired ion therefrom by means of a mass spectrometer, and scanning and guiding the selected ion to the target at an appropriately decelerated speed. In this way a metal thin film is formed in a desired pattern on the target, the metal film having a line width in the order of submicron. This method is advantageous in that the resulting film is safe from impurities, thereby enhancing the quality of the film.
Referring to FIG. 8, the apparatus used for performing the known methods will be described:
The focused ion beam apparatus includes a liquid metal ion source 81 for storing a suitable metal, and a target stage 87, wherein the source 81 and the stage 87 are located at opposite ends of the apparatus. Between the source 81 and the stage 87 are electrodes 82, a mass spectrometer including an electrostatic lens 83a and a mass filter 84a, an electrostatic lens 83b, a single stage of deflection electrodes 85, and a decelerating electrode 86 interposed.
An ion beam extracted from the ion source 81 at an accelerated speed is focused by the first electrostatic lens 83a, and a desired ion species is selected and separated by the mass spectrometer 84a and 84b. The separated ion beam is focused by the second electrostatic lens 83b (hereinafter referred to as "objective lens") and decelerated to a desired level of energy by a decelerating field existing between the decelerating electrode 86 and the target W before it reaches the target W.
The ion beam is deflected by controlling a voltage applied to the deflection electrodes 85 interposed between the objective lens 83b and the decelerating electrode 86.
The conventional focused ion beam apparatus described above has disadvantages in (1) that the objective lens 83b are unavoidably located at a relatively long distance from the target stage, thereby enlarging the diameter of a beam spot formed on the target W, and (2) that since the beam is deflected after it is focused by the objective lens 83b, aberration is likely to arise and expand the diameter of ion beam particularly when the ion beam deflects at an excessively large angle. These factors (1) and (2) prevent the beam spot on the target from being focused as intensely as desired. Thus it is difficult to achieve a fine, delicate pattern.
In addition, the known focused ion beam apparatus is disadvantageous in that a scanning ion microscope (SIM) cannot be used to direct the beam exactly to the target by watching the beam position unlike the ion beam assisted CVD method. In contrast, the ion beam assisted CVD method is advantageous in that the same ion beam can be used for twofold purposes; one is to watch the target by detecting secondary electrons emitted from the target, thereby facilitating the positioning of the ion beam with respect to the target, and the other is to perform the CVD method.
The focused ion beam direct deposition method is disadvantageous in that the decelerating field provided to slow down the focused ion beam immediately before the target tends to push back secondary electrons toward the target. As a result, the secondary electrons fail to reach the SIM detector, thereby making it impossible to use the focused ion beam decelerated for deposition to locate the ion beam against the target. In this method, therefore, it is required to project the ion beam against the target without preparing the decelerating field. Nevertheless, the ion beam is adversely affected by the decelerating field which is provided for performing the ion deposition; that is, the paths of the non-decelerated ion beam and the decelerated ion beam come into disaccord with each other because of the difference in speed. If a pattern is determined on the basis of an SIM image, the resulting pattern tends to become larger than expected because the ions flow at a decelerated speed for a prolonged period of time.
The direct deposition method requires a sufficiently strong decelerating field to prevent the ions from dispersing but any mechanical dislocation of the electrode tends to dislocate a pattern to be deposited.